Rayleigh's Horizon

The Science

The sky in this photo is a live demonstration of Rayleigh scattering. Sunlight looks white but is actually every colour mixed together, and when it hits the gas molecules in our atmosphere, the shorter wavelengths (blue and violet) get scattered far more strongly than the longer ones (red and orange). At sunset, the sun sits low, so its light has to travel through a much longer stretch of atmosphere to reach me. Along that long path almost all the blue is scattered away before it arrives, and what's left is the band of deep red and orange glowing along the horizon. Higher up, the sky is still blue because I'm seeing light scattered from the thinner air above. There's a second effect visible here that comes from being at altitude. The dark blue-grey band sitting above the bright horizon is the edge of Earth's own shadow being cast up onto the atmosphere as the sun drops below the horizon, the start of twilight, seen from above. The aircraft wing appears as a flat silhouette for a simple reason: the sky behind it is far brighter than the shaded underside of the wing, so the camera exposes for the sky and the wing goes dark. One frame, taken from a plane window, holding both the physics of why sunsets are red and the geometry of the planet turning away from the sun.

Why I Chose It

I chose this photo because it captures two different pieces of physics in a single frame, the Rayleigh scattering that turns the horizon red and the edge of Earth's own shadow rising into the twilight. It's a reminder that not all scientific processes require a lab, but rather are found around us.

Eshal Zeeshan Verified Volunteer

Volunteer · Explorer · Lahore · Member since Jun 2026